Does Antimatter Fall Up?
May 7, 2013

Does Antimatter Fall Up?

The interaction of matter and gravity is so pervasive that we hardly even notice. Throwing a ball into the air will cause it travel in a parabolic arc, influenced by Earth’s gravitational field. Satellites, the International Space Station, and even the Moon are bound in orbit around the Earth, held there by interactions of gravity.

But what if this was not always the case? There exists a form of matter that we know as antimatter, essentially the opposite of the matter that composes virtually everything that we see around us.

These particles and nuclei have the opposite charge, the opposite spin, yet the same mass as their “normal” counter parts. The electron has the position, and the proton has the anti-proton, and so on. And while these anti-particles have been well studied, there are still some questions surrounding their properties.

Since antimatter particles have the opposite quantum properties from normal matter, might it also make sense that there is also something “opposite” about its mass? We already know that the absolute mass is the same, that it is not “negative” mass (though construct is not necessarily prohibited by physics).

But what if the difference arose in how the mass interacted with gravity? What if, instead of antimatter falling toward a massive body, as normal mass does, antimatter falls up?

It turns out that measuring such an effect is difficult; dealing with antimatter takes special care since it cannot be allowed to interact with normal matter. (When matter and its antimatter counterparts collide they annihilate into photons.) But a new study using data from CERN’s ALPHA experiment has started investigating this very question.

Using 434 antihydrogen atoms (made up of a position orbiting an anti-proton), the team searched for differences in the predicted particle speeds as anti-atoms flew through their apparatus.

What they found was inconclusive. The variation in the atomic speeds was within the margin for error that they were able to detect with their current set-up. But a forthcoming upgrade, known as ALPHA-2, will allow much greater precision in the measurements, and should reveal if there is a difference in how antimatter interacts with gravity.

“Is there such a thing as antigravity? Based on free-fall tests so far, we can’t say yes or no, “ reports Joel Fajans of Lawrence Berkeley National Laboratory. “This is the first word, however, not the last.”

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